Molecular vacuum pump



Jan. 2, 1934. DuB N 1,942,139

MOLECULAR VACUUM PUMP Filed Dec. 26, 1930 3 Sheets-Sheet 1 Jinan/towJo/UL DLLbPOUL/ Jan. 2, 1934. DUBRQVIN 1,942,139

MOLECULAR VACUUM PUMP Filed Dec. 26, 1930 3 Sheets-Sheet 2 iii/@7770]70%]2 DuPOUZ/Z Jan.- 2, 1934. J. DUBROVIN MOLECULAR VACUUM PUMP 3Sheets-Sheet 3 Filed Dec. 26, 1930 UNITED STATES PATENT QFFECE MOLECULARVACUUM PUMP John Dubrovin, Chicago, 111., assignor to Central ScientificCompany, Chicago, Ill., a corporation of Illinois Application December26, 1930 Serial No. 504,715

9 Claims.

This invention relates to improvements in molecular vacuum pumps.

The objects of this invention are to produce a practical, effective andmore eflicient pump; to produce a pump having a compact design, acheaper construction and a greater simplicity of manufacture.

These and other objects and advantages will become apparent from thespecifications and the appended drawings in which Fig. l is a verticalsection through the pump showing the motor, however, in elevation;

Fig. 2 is a section on the line 2--2 of Fig. 1 and indicates by dottedlines the relative position of the two spirals;

Fig. 3 is a section through the upper plate on the line 3-3 of Fig. 1;

Fig. 4 is a section through the device on the line 4-4 of Fig. 1;

Fig. 5 is a section through the device on the line 55 of Fig. 1;

Fig. 6 is a vertical section through a portion of a modified form ofpump;

Fig. 7 is a plan view on the line 7-7 of Fig. 6; and

Fig. 8 is a plan view on the line 8-8 of Fig. 6.

The pump comprises a cylindrical casing 10 provided with a top flange11, and a bottom flange 12, cooling fins 13, 13 and a motor supportingportion 14. The casing is closed by the top cap 15, and the bottom cap16, each of which carries a flange 1'7 and. 18, respectively, which aredrawn tight in companion flange arrangement against the casing flanges11 and 12. The cap 15 is held by the bolts 19. On the lower flange,certain of the bolts are replaced by the cap screws 21, which are tappedinto the legs 22. The casing is made vacuum tight by the circulargaskets 23, 23, which fit in grooves 24, 24, cut into the upper andlower caps. For certain extreme service conditions, the gasket isomitted and the grooves are connected to a separate vacuum pump. Withthis arrangement proper sealing agents may also fill the groove.

Held to the flange 11 by the screws 25 is an assembly consisting of alower plate 26, which bears upon the inner portion of the flange 11, aspacing ring 27 and a top plate 28, these together forming the statorelement of the pump. A disc 29 mounted between the two plates forms therotor element. The clearance between the plates and disc and disc andring (in one place) is exceedingly small (.0015 to .002 inches), whichrequires that all the surfaces, including the flange 11 be ground andfinished with precision.

The plates may be of any metal; I prefer cast iron, but because of thehigh speed, the rotor 29 should be as light as possible and I find itvery advantageous to make this from either hard aluminum or magnesiumalloys.

The portion 14 of the casing is bored out to receive the fieldlaminations 31 of a series type motor 32, which are forced home underconsiderable pressure. The motor itself forms no part of this invention,consequently, its details are not shown. It is, however, provided with aspecial step bearing (shown cased at 33) which adapts it to run in avertical position. The disc 29 is held on the conical portion 34 of themotor shaft 35 by the Whitworth keys 36, 36 and the Z0 nuts 37, 37. Theusual oiling means for a motor are not used. Instead the small tubes 38,38 conduct grease to the bearings. The tubes are carefully sweated intothe nipples 39, 39, which are tapped into the casing 10. After the tubeshave been filled with grease, the caps 41, 41 are screwed down over thenipples until the gaskets 42, 42 engage the flat outer face of thenipple. This system of lubrication is effective and vacuum tight.

When the pump is in operation, there can be no ventilation of the motor,since the atmosphere around it is usually exhausted to below one micron.I find that the long thermal contact between the field laminations andthe casing is the most efiective means of cooling the motor and the fins13 are provided to radiate the heat as quickly as possible. They alsomaterially stiffen the casing, a very necessary precaution, since thetotal clearance in the device is less than 90 flve thousandths of aninch. 7

Current is led into the casing through two bushings 43. They comprise asteel fitting 44 having a taper threaded nipple 45 and an internal bore46 terminating in the taper tapped portion 47. A sleeve 48 of hardrubber is screwed into the bore and a plug 49 of brass which also bearsa taper threaded portion is screwed into the sleeve. I prefer to coverthe bushings with the protective cap 51. The lead 52 is connected to theplug by the screw 53 while the motor lead 54 is prevented from shortingby the washer 55 and held connected to the plug by the nuts 56. I havefound that hard rubber possesses definite advantages in producing avacuum seal. It seems to freeze both to the steel and the brass. It isonly with the greatest difficulty that the bushing can be made vacuumtight if a phenolic sleeve is used.

The intake and outlet for the pump are through the space 57 and. thebore 58 of the nipple 59,110

respectivelyl It is essential that a backing pump be used. Usually it issufficient to connect this to the nipple by means of a rubber hose. Forthe highest vacua, all glass connections should be made between thevessel to be exhausted and the molecular pump. In that case, themanifold tube should have an integral glass flange ground flat to matewith the flange 61 which surrounds the intake, and be sealed theretowith vacuum wax. Rubber connections are a great convenience and areuseful in short experiments. The flange 62 carrying the rubber tubenippie 63 is consequently provided which may be pulled tight against thegasket 64 by the cap screws 65.

A bafiie or guard ring 66 surrounds the central aperture in the plate 28to prevent any possibility of broken glass or other material workingbetween the rotor and the plates.

Before discussing the gas path through the device, it is necessary toexamine the plates 26 and 28 and the spacing ring 27.

Fig. 2 is a view showing the lower portion of the plate 28 and (dotted)the elements below it. It will be seen that the plate bears a wide,shallow channel 67, shaped as a one turn spiral. Its depth should belarge in comparison with the clearance between plate and disc and inthis instance is about 3 millimeters deep. The plate also bears a largecentral aperture 68 forming the inlet passage to the pump elementsproper.

The lower plate 26 bears a similar but reversed spiral channel 69 and alarge central aperture 71 forming the outlet passage from the pumpelements proper. Between the disc 29 and the spacing ring 27, there is acircumferential channel 72 whichis blocked over a small sector by anintegral projection from the ring forming the barrier 73. Between thebarrier and the disc 29 the clearance is as small as is commerciallypractical.

I The path of the gas through the device is shown by the arrows. Itenters the intake 57, passes 'over the baffle 66, through the centralorifice 68 in the plate 28, into. the spiral channel 67. From there, itis flung against the walls of the spacing ring 27 in the channel 72 andcarried along by the cylindrical face 74 of the disc 29 until it'r.eaches the barrier 73. It then enters the spiral 69 and travelstherein until it passes through the central orifice 71 in the plate 26and out into the backing pump through the exhaust passage 58.

In the modification shown in Figs. 6, 7 and 8, the conducting channelsare placed in the disc rather than in the plates. The disc 74 has thespiral channel 75 formed in its face, and a like, but reversed channel76 in its lower face. The plates 77 and 73 present plane surfaces. Allother features of the pump remain the same. In

operation, this latter pump appears to duplicate the performance of thepreferred form, but is more difficult to build since to form oppositelydirected spirals in the disc and still have it remain balanced at 18,000E. P. M. requires very careful work.

For a clear understanding of the function and nature of the improvementswhich I have herein set forth, the difference between molecular pumpsand those bearing a certain mechanical similarity, namely centrifugalpumps and viscosity pumps, must be clearly held in mind.

In a centrifugal pump, the particles of the pumped mass are pressedagainst the rotating element by some force, usually atmosphericpressure, and as the particle slides out radially across the rotor inresponse to the centrifugal force,

some of the velocity and energy of the periphery of the rotor isgradually imparted to it.

In a viscosity pump the liquid or gas filling the space between therotor and stator is subject to shearing stress since the fluid tends toadhere to the pump elements and, in yielding to the stress, builds up apressure differential between the intake and the outlet of the pump.

In a molecular pump, a free moving molecule strikes against the surfaceof a rapidly whirling element. Some adhere; more bound off in thedirection of the resultant force and if a conduit confine their;movements, the molecules will bound off the walls, hit the movingelement again and generally be driven along in the desired direction. Byno means do all the molecules follow auniform path, nor do all of themprogress with the rotor but the statistical result is a pronouncedmovement toward the outlet and a pressure differential corresponding tothe kinetic energy of the mass flow.

Taking one gas to be specific, the average mean free path of a moleculeof nitrogen at atmospheric pressure is given as 9.4=4=x10- cm. When ithas travelled through this microscopic distance, the molecule willcollide with another. In comparison with the numbers of molecules in thespace at this pressure, the chances of many travelling far enough to hitthe moving wall are remote. Also those thatare displaced are retarded bythe cloud of molecules in the space. A molecular pump at such apressure, though workable, is ineffective. The average mean free path ofa molecule of nitrogen at one micron pressure is, however, 71.7centimeters and its average velocity at 20 centigrade is 496 meters persecond. One micron is a pressure which is easily maintained by a goodbacking pump. In the incredible number of collisions that result, it maybe assumed that nearly all the molecules will sometime 201- lide withthe moving element. Only the moving area propels the molecules ahead,consequently, the more surface area of the conduit occupied by themoving part, the better is the pump.

It follows that the pump reaches its greatest e efficiency when theratio of the moving surface per unit length of conduit to the crosssectional area of the conduit is as great as practicable. This limit isset by the requirement that the depth of the channel must be large incomparison with the clearance between plate and disc, the limiteddimensions of the disc and the certain length of channel that isnecessary.

It also follows that, keeping all other conditions constant, the longerthe path through the pump, the more effective the pump becomes. It istrue also that the efficiency of the pump will rise with increasingmotor speed. Whether or not an increase of speed beyond 18,000 R. P. M.is of practical value seems to me questionable, but. even at 18,000 R.P. M. tremendous rim speedexists which rigidly limits the diameter ofthe rotor. The advantage of using both sides and the periphery of therotor is now apparent. I am able to-se'cure sumcient length and also awide and shallow f" channel without increasing the diameter of the.rotor to a point where gyroscopic 'eifects are stage in the exhaust,this pump seems to take hold and the pressure thereafter falls rapidly.The low limit of this pump is below the sensibility of the usualmeasuring devices and so far, I am familiar with no method which willdetermine its ultimate performance with accuracy or precision.

The particular embodiment shown has been chosen by way of illustrationonly. For example, cylinders might be substituted for the rotor andplates. If the benefit of the long circumferential path around theperiphery of the rotor is foregone, other arrangements suggestthemselves, Viz, the two faces of the disc might be operated inparallel, or if a passage through the disc near its center wereprovided, the intake and outlet might be at the periphery, or otherarrangements and changes might be made without departing from the spiritof the invention herein set forth.

What I claim, therefore, is:-

1. A molecular pump comprising a casing havin a flared upper portion anda cylindrical bore portion, a motor positioned within the bore andhaving its field laminations in close contact with the interior Wall ofthe bore, two plates maintained in spaced relation upon the casing, adisk rotatably mounted between the two plates, said plates having gasconducting channels upon their opposed faces and a connecting channelextending about a major portion of the periphery of said disk, a shaftfor said motor connected to said disk for rotating the same, said shaftextending through an enlarged unobstructed opening in one of saidplates, caps closing the ends of the casing and an inlet to and outletfrom the pump, said outlet being located in the flared upper portion ofsaid casing.

2. In a molecular pump, a casing, a stator member within said casing, aninlet passage for said casing, a rotor member rotatably mounted in saidstator member, a shaft for operating said rotor member, the bearings forsaid shaft being supported independently of said stator member, one ofsaid members having a spiral groove therein, said stator being providedwith an opening oifset from said inlet for conducting gas from saidcasing to said groove and a baffie surrounding said opening.

3. A molecular pump comprising a casing having a flared upper portionand a cylindrical bore portion, a motor positioned within the bore andhaving its field laminations in close contact with the interior wall ofthe bore, two plates maintained in spaced relation upon the casing, adisk rotatably mounted between the two plates, said plates having gasconducting channels upon their opposed faces and a connecting channelextending a material distance along the periphery of disk for conductingmolecules of gas along the the connections between the grooves and thechannel being spaced angularly apart around the periphery of the disk,caps closing the end of the casing and an inlet to and an outlet fromthe pump.

4. In a vacuum pump, a casing, a pair of spaced stationary elementswithin the casing having a shallow, Wide spiral groove formed in theiropposed faces, one spiral having a clockwise direction, the opposingspiral having a counter clockwise direction, a rotatable member betweenthe elements, said rotatable member having a peripheral face, and apassage concentric with said peripheral face and extending about a majorportion of said rotatable member and connecting at its ends with theends of the two grooves for conducting gas along the periphery of saiddisk and from one of said grooves to the other.

5. In a molecular pump, a casing having therein a pair of plates spacedapart and provided with open channels, a disk rotatably mounted betweenthe plates, a passage having its opposite ends connecting the channelsin the two plates, said passage extending about a major portion of theperiphery of said disk, one of the plates having an aperture to delivergas to the central portion of the disk and the other plate having anaperture to receive gas discharged by the central portion of theopposite face of the disk.

6. In a vacuum pump, a casing, a pair of spaced plates within the casinghaving gas conducting channels in their opposed faces, a ringmaintaining the plates in spaced relation, a rotatably mounted diskhaving a cylindrical face between the plates and within the ring andhaving a sensibly less diameter than the bore in the ring to provide anannular gas passage between the cylindrical face of the disk and thering, an inwardly extending sector in the ring closely approaching thecircumference of the disk forming a barrier in the annular passage, andmeans connecting each of the gas conducting channels separately with theannular passage at opposite sides of the inwardly extending sector.

7. In a molecular pump, a casing, a stator member within said casing, aninlet passage for said casing, a rotor member rotatably mounted in saidstator member, one of said members having a spiral groove therein, saidstator being provided with an opening offset from said inlet forconducting gas from said casing to said groove and a baffle surroundingsaid opening for preventing entrance of foreign matter into the pump.

8. A molecular pump comprising a stator member having a pump chambertherein, a rotor member mounted within said chamber, means for rotatingsaid rotor member, said rotor member being in the form of a disk havingits faces in proximity to the walls of said chamber and having itsperiphery spaced from the peripheral wall of said chamber to form apassage extending a major portion of the distance around said disk, oneof said members having spiral channels at opposite sides of said diskfor conducting molecules of air across said disk at one side thereofinto one portion of said passage and for conducting said molecules fromanother portion of said passage back across said disk at the other sidethereof during the operation of said pump.

9. A molecular pump comprising a casing, two plates spaced apart withinthe casing, a disk rotatably mounted between the two plates, an intakeand discharge for said pump, a passage extending a major portion of thedistance about the periphery of said disk, said disk having

